As the star of Zero Dark Thirty, Jessica Chastain was the center of attention at a lunch thrown in the film's honor in Manhattan last Thursday, days after the National Board of Review named her the year's best actress, and the movie, directed by Kathryn Bigelow, 2012's best film. She shook hands and chatted with Academy members, industry players and colleagues from the picture -- coming full circle in a personal and professional journey that began over a decade ago.

Chastain was in New York on Sept. 11, a Juilliard graduate still learning to tap in to and channel her emotions; 11 years later, the Oscar nominee and Broadway star says she had to both utilize and overcome that training to perform what is biggest role of her career.

The film, which tells the story of the hunt for Osama bin Laden, begins early in America's war in Afghanistan. In a dark warehouse, Chastain, as a young CIA agent named Maya, witnesses a fellow intelligence officer named Dan (Jason Clarke) torturing an al-Qaida courier in an effort to procure the location of an important terrorist. Dan waterboards the courier, ties him up and crams him into a box, doling out mental abuse all the while. It is a practice that America has banned, and it's a bracing opening for the film.

"It was really difficult to film even though, of course, we’re acting," Chastain told The Hollywood Reporter at the Midtown lunch. "But we filmed it in an active Jordanian prison. The energy wasn’t the best in that place. I’m playing a woman who’s trained to be unemotional and analytically precise. I’ve been trained my whole life to be emotional and to let all my walls down and be very vulnerable. So to put myself in a situation like that, it’s like I have to not follow my instincts -- and my instincts, it seemed like that would be to cry. So I had to then show her discomfort, but to go back to her training of being unemotional."

Making it even more difficult was the fact that Chastain was on her own in that process. She never met the dogged CIA agent on whom screenwriter Mark Boal and Bigelow based her character; as an undercover operative, the woman was unavailable for questioning. Both the film's script and the research Chastain was given offered little personal information about the agent, leaving the petite, redheaded actress, who likes to prepare for months for her roles, with many blanks to fill.

"Anything I couldn’t find in the research about Maya, I then used my imagination to create," she explained. "Like silly things, like what her favorite candy in America would be. You know the scene where I’m wearing the robe and I’m eating candy? Stuff like that, so I could answer questions. I also had to create a backstory. You see in the movie, you see there’s a drawing by a child. You see postcards. All these things that I had created from this history of this woman that would make sense and go along with the research I had made of her."

Clarke -- whose character, a long-haired, tattooed tough man with a Ph.D., was also based on a real agent -- did plenty of of his own research.

"You can read a lot of these on the Internet," he said of the torture methods he used in the film; he also went through some simulated waterboarding of his own. "You’d be surprised what’s on there ... The techniques and the devices they use are factual, and they’re there and they’re not that complicated. I’ve had a couple of big nights out. I know what it’s like to not sleep for 28 or 48 hours. But what I was interested in was creating a relationship."

"A good interrogator was always able to keep the conversation going, keep it open," he continued. "Because that’s what you’re after. To allow someone, even like a therapist, to unburden them. To make sure that I’m here for you because it’s what we both want."

With five months shooting on location in places such as Jordan and India, there were plenty of opportunities for on-the-ground investigation, and like a real CIA agent, the Australian actor spent an extensive amount of time in the field, picking up the spirit of the foreign cultures.

"I like to find things to do. That’s part of my character research, too," Clarke said with a smile. "I'll go to the local bathhouse and tell them I’m a student hanging out, meet some people, take someone out for some falafel. What’s going on in the country? What’s the word on the ground? That’s what these CIA dudes do; they got to move in there, they’ve got to take a job that’s a pretend job, but make sure that it’s 'real.' And just blend in."

Back in the United States, he had discussions with ex-Navy SEALs and intelligence officers who now serve as advisors to the film industry, and got to know other former soldiers while surfing with them in San Diego. It was eye-opening, Clarke said, to learn just how much the military depends on the instincts of individual agents and soldiers.

Chastain's research was informed by real-life experiences as well, including time she spent in New York years after graduating Juilliard.

"I was here for a few days when the news came that [bin Laden] had been killed, and I remember when the waiter came and told us -- I was at a table with a group of friends -- told us what had happened," she recalled, growing somber. "It wasn’t like a fist-pumping, go-America type of feeling. It was more this shock, it was emotional, it was the end of the circle of a very long decade, intense decade for us, but also it didn’t feel like the ending. It felt like the end of a chapter but the beginning of a new chapter, and I didn’t exactly know what it meant."

Months later, she would get to help tell that chapter's untold story to the world.